BMWs are pretty much always classy and have a timeless design (with the exception of the E83 X3. I hate that stupid looking thing. WHAT THE HELL WERE YOU THINKING BMW?)
In terms of previous 3 series platforms, the e30, e36, etc. are still beautiful looking cars to this day. The reason why they are forgotten and we don't see them as much on the road is because the majority of BMW owners today consider a BMW as nothing more than a status symbol. I'll bet you 90% of the time when a person purchases a BMW, the only thing they think about is status. They don't care about performance numbers or the driving dynamics, they just want to tell the world that they are successful and have money. And obviously to show the world that you are rich and successful, you have to have a new shiny car. No status buyer would dare own a pervious generation 3 series because they fear that owning an older car will lead others to classify them as a lower class. This is what makes older generation cars seem unattractive and forgotten to most people. The car itself is not bad looking, people make them out to be due to their negative impact on one's perceived social class thus making them forgotten.
To me personally, I will never forget about my e92 M3. I didn't buy it so I could show off to people. I bought it because I love driving it. There are so many things that I love about it such as the high revving V8, the linear power delivery, the styling, etc. I will replace it when I find another car that gives me the same amount of joy or more. It could be the new M3 or the M3 after that or some other car. But I am sure that I won't be buying the next gen M3 just because it will be the next gen M3.